1. Field of the Invention.
The invention relates to the generation of video color signals from digital signals.
2. Prior Art.
Over the past twenty years or so, there has been a requirement to interface the commonly used raster scan cathode ray tube to the digital outputs of a digital computer. An early example is shown in U.S. Pat. No. 3,345,458 which describes a system for digitally generating characters for a raster scan display.
In more recent years there was a need to provide an interface between a digital computer and a standard (NTSC) color display. The NTSC system relies on phase differences of analog signals which are not easily made compatible with digital signals from a computer. A circuit for converting digital signals to a direct composite color signal is described in U.S. Pat. No. 4,278,972. In this sytem, digital signals representing colors are shifted in a recirculating shift register in synchronous with the color reference signal (3.58 MHz). This system provides the composite video used in the Apple II series computers.
A more recent need has been to generate parallel color control signals such as those used in a red-green-blue (RGB) monitor from the digitally generated composite video. The RGB monitors have become more popular as their price has declined and since they provide better color displays. The composite video signal from, for example, an Apple II computer must be converted before it can be used for an RGB monitor or a color printer.
There is a commercially available apparatus described in conjunction with FIG. 1 for converting the composite color video generated by, for instance, the Apple II computer to parallel color control signals for an RGB monitor. The problems with this current apparatus will also be described in conjunction with FIG. 1.